High dynamic range digitization systems provide many advantages when used in sensor measurement devices, such as ultrasonic flaw detectors, because they solve many of the problems associated with devices using analog intensive solutions. The advantages are described in detail in Thomas US 2007/0084288 A1 and summarized below.
Typically, ultrasonic flaw detectors that use high frequency sampling rate analog to digital converters in the range of 100 MSPS (million samples per second) are limited to 14 bits resulting in a maximum dynamic range of only 84 dB, an amount less than required by industry standards, and many sensor measurement applications. Background art solutions having only one analog to digital converter achieve higher dynamic range by using one or more variable gain amplifiers (VGA's), but not without significant problems.
The primary problems are: 1) the need for a large number of analog filter components and the noise, power, reliability, and size problems that go along with them; 2) the DC offset compensation required to keep the signal centered within the full scale range of the system as the gain changes is difficult to implement because it must be calibrated and applied dynamically, and 3) it is impractical to apply the advantages of digital filtering because the VGA applies a variable low pass filtering effect that must be accounted for in the digital filter, which adds much complexity to the filter system.
Accordingly, a means to meet or exceed the dynamic range of the background art system by replacing the analog variable gain function with a digital one is desirable. As taught in Thomas US 2007/0084288 A1, two or more high MSPS analog to digital converters may be used to achieve higher dynamic range to solve many of the problems associated with the analog intensive background art solutions. There is, however, a new problem created by this means that needs to be solved before optimal performance can be achieved—i.e. the problem of sample errors caused by phase and amplitude errors between adjacent scaled analog to digital converter channels.
Each embodiment of the present disclosure provides a means to solve this new problem. As will be explained in detail later in the present enclosure, the sample errors are undesirable because: 1) they degrade the signal image that is observed by the instrument operator when making inspection judgments, and 2) they add distortion to the sensor input signal, thereby increasing the likelihood of measurement errors. The Assignee of the present patent application has filed several U.S. patent applications directed to an ultrasonic fault detection system using a high dynamic range analog to digital conversion system, which published under U.S. Patent Application Publication Nos. 2007/0084288, 2009/0178485, and 2009/0223294, and the contents of said published patent applications are incorporated by reference herein.